The Zika virus can be transmitted through sex, which is a bit chilling if you think about it: Not only can the virus pass through sexual partners but those encounters could, in theory, result in babies born with birth defects.
It’s this fact that has people worried that Zika could become the devastating STD of the decade. An op-ed in the New York Times recently called Zika "the millennials’ STD." The virus was also recently compared to HIV, one of the biggest killer viruses of our time.
I asked several researchers who are studying Zika about its potential to become a widespread STD. Their main conclusions: It’s still too early to tell, but probably not. The biggest reason why it's unlikely is people are still far more likely to get Zika from mosquitoes than through sex.
Let's also recall that there’s a lot we still don’t know about this virus. As early as last year, all the world’s knowledge of Zika fit inside a shoebox. Back then, we weren’t even certain that Zika was sexually transmitted, or that it caused birth defects like microcephaly. Now we are.
Entire world literature on Zika. 50 years of neglect. pic.twitter.com/iX7zbJSTC5— Dr. Tom Frieden (@DrFriedenCDC) February 12, 2016
Even so, there are a couple of reasons why researchers don’t think sexual transmission will be an important way Zika spreads compared with those "syringes with wings" (a.k.a. mosquitoes).
The first is that mosquitoes are very, very efficient vectors — unlike humans.
"A single mosquito can take multiple blood meals throughout the day," said Nikolaos Vasilakis, a professor and researcher at the University of Texas Medical Branch. During those blood meals, an infected mosquito would release saliva containing the virus into its victims, which can potentially lead to Zika infections.
Compare that with Zika spread through sex: Very few people will have multiple sexual partners in a day (or day after day). "Yes, sexual transmission may play a role [in spreading the virus]," Vasilakis said, "but not as significant as the bites of a mosquito, which can go serially within a short period of time."
The second point about Zika: Once people get the virus, they have what researchers think may be lifelong immunity. This makes Zika unlike the bacterial STDs gonorrhea and syphilis, which people can get multiple times throughout their lives.
This is important because of a concept called "herd immunity," explained Albert Ko, a Yale University epidemiologist and infectious diseases specialist. "Once enough people are exposed to the virus, it protects others who have not yet been exposed," he said. So if the virus has already spread within a population, it’s unlikely to cause a big outbreak again. "Is Zika going to be the new STD for millennials?" Ko said. "I would suspect not."
There's still a lot to learn about Zika
Again, there’s a lot the scientific community has yet to learn about Zika. For instance, it’s not clear how long people with the virus remain infectious and can spread it through sex.
Zika has been documented in semen six months after an infection — but that study only identified evidence of the virus, and and not whether there was enough to actually cause an infection in another person.
According to the European Center for Disease Prevention and Control, "viable Zika virus" has been detected in semen up to 24 days after onset of symptoms. "The longest interval reported between the onset of symptoms in an infected man and the subsequent onset of disease in a female sexual partner is between 34 and 41 days," the ECDC wrote — so that’s much less than six months. (In women, researchers have found Zika in vaginal fluid 11 days after a woman showed symptoms of infection — but female-to-male Zika transmissions have been rarer, so there’s more uncertainty about them.)
It's also unclear how many sexually-transmitted infections originate with someone who has the virus but no signs of it. Researchers this week identified a case involving a Maryland man who had symptomless Zika and who infected his sex partner — the first time this type of transmission has been documented.
"I do think it is premature to conclude that Zika is the next STD epidemic," said Duane Gubler, a leading researcher of mosquito-borne diseases at Duke-NUS Medical School. "Having said that, I do think we should err on the side of caution until we do know more."
Right now, health authorities recommend that men practice safe sex or even avoid sex for up to six months after a Zika infection, and that women who have come into contact with the virus wait at least eight weeks before trying to conceive.